1. Field of the Invention
The present invention relates to a honeycomb catalyst body. More specifically, the invention relates to a honeycomb catalyst body having zeolite and vanadia loaded thereon and usable for selective catalytic reduction (SCR) of NOx.
2. Description of Related Art
An exhaust gas discharged from internal combustion engines including automotive engines contains harmful substances such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx). For reducing such a harmful substance and thereby purifying an exhaust gas, a catalytic reaction has been used widely. In gasoline engines, a ternary catalyst has been used generally which maintains a mixing ratio (air fuel ratio) of air and a fuel at a theoretical air fuel ratio, bringing CO, HC, and NOx in an exhaust gas into contact with a noble metal catalyst such as platinum or rhodium to convert them into harmless CO2, H2O, and N2.
A ternary catalyst is not suited for use in diesel engines because in general, an amount of air is excessive relative to the amount of a fuel and the air fuel ratio cannot therefore be maintained at a theoretical air fuel ratio. An oxidation catalyst for reacting CO and HO with O2 in the excessive air to convert them into harmless CO2 and H2O can be used, but it cannot reduce NOx into N2 in an oxygen-excess exhaust gas atmosphere of diesel engines. For diesel engines, therefore, a countermeasure against NOx has been a big issue to be solved.
Examples of a technology of reducing NOx in an oxygen-excess atmosphere include selective catalytic reduction (SCR). Selective catalytic reduction (SCR) is a technology of reacting NOx with ammonia (NH3) and thereby converting it into N2 and H2O and it has conventionally been used as an exhaust gas treatment system in thermal power plants and the like. In recent years, application of a technology making use of selective catalytic reduction (SCR) to diesel engines for vehicles has been underway. Loading of NH3 on vehicles is however dangerous so that reduction of NOx is conducted by loading a tank filled with an aqueous urea solution on the vehicles, injecting the solution into the exhaust gas to hydrolyze it at a high temperature and obtain an NH3 gas, and using the resulting NH3 for the reduction.
In the exhaust gas treatment system in thermal power plants and the like, a titania-vanadia catalyst is used for reacting NOx with NH3. The titania-vanadia catalyst has two types, that is, coat type and solid type. Similar to ternary catalysts or oxidation catalysts for automotive engines, the coat type is obtained by loading titania and vanadia on a ceramic honeycomb carrier. The solid type is obtained by forming titania and vanadia themselves into a honeycomb structure. In recent years, most of the titania-vanadia catalysts have been a solid type. Although catalysts similar to those used in an exhaust gas treatment system of thermal power plants and the like can also be used for diesel engines for vehicles, the titania-vanadia catalyst require a large volume in order to achieve highly-efficient NH3 purification. Using the titania-vanadia catalyst for vehicles is not recommended. There is therefore a demand for a catalyst for vehicles that can be down sized and has a higher efficiency.
Catalysts more efficient than titania-vanadia catalysts include metal-substituted zeolites (such as copper ion-exchanged zeolite and iron ion-exchanged zeolite). Similar to titania-vanadia catalysts, the metal-substituted zeolite catalysts have a coat type and a solid type. Catalysts for vehicles are fixed in a metal container via a heat-resistant cushioning material. In order to prevent catalysts from moving due to vibration of vehicles, catalysts are retained by a friction power while applying a compressive force. The catalysts should therefore have mechanical strength enough to withstand the compressive force. The coat type (zeolite-loaded honeycomb catalyst body) does not have a large problem in strength because a ceramic honeycomb carrier has mechanical strength. In the solid type (zeolite structural body), on the other hand, a honeycomb structure should be made of zeolite. An inorganic binder for binding zeolite particles to each other or an aggregate or fiber for keeping its strength should be added to zeolite (for example, Patent Documents 1 and 2).
[Patent Document 1] JP-A-2012-213755
[Patent Document 2] JP-A-2011-207749